For liquid crystal display elements, the properties of nematic or nematic-cholesteric liquid crystal materials are utilized to effect a significant change in their optical properties, such as light transmission, light scattering, double refraction, reflectance or color, under the influence of electrical fields. The function of such display elements is based, for example, on the phenomenon of dynamic scattering, the deformation of oriented phases, the Schadt-Helfrich effect in the twisted cell or the nematic-cholesteric phase transition.
For industrial application of these effects in liquid crystal display elements, liquid crystal materials which must meet a large number of requirements are needed. Particularly important requirements are chemical stability towards mositure, air and physical influences, such as heat, radiation in the infrared, visible and ultraviolet ranges and steady and alternating electrical fields. Furthermore, a liquid crystal mesophase in the temperature range from at least 0.degree. C. to +50.degree. C., and preferably from -10.degree. C. to +70.degree. C., and a viscosity at room temperature of not more than 60 cP are demanded for liquid crystal materials which can be used industrially. Finally, these materials must not have any characteristic adsorption in the visible range, that is, they must be colorless.
A number of liquid crystal compounds which meet the stability requirements demanded of dielectrics for electronic components and which are also colorless are already known. These include, in particular, the p,p'-disubstituted phenyl benzoates described in German Offenlegungsschrift No. 2,139,628 (U.S. Pat. No. 4,002,670), the p,p'-disubstituted biphenyl derivatives described in German Offenlegungsschrift No. 2,356,085 (U.S. Pat. No. 3,947,375) or the phenylcyclohexane derivatives described in German Offenlegungsschrift No. 2,636,684 (U.S. Pat. No. 4,130,502).
In these categories of compounds and also in other known series of compounds having a liquid crystal mesophase, there are no individual compounds which form a liquid crystal nematic mesophase in the required temperature range from 0.degree. C. to 60.degree. C. Therefore, as a rule, mixtures of two or more compounds are prepared in order to obtain substances which can be practically used as liquid crystal dielectrics. For this purpose, usually at least one compound having a low melting point and clear point is mixed with another compound having a significantly higher melting point and clear point. This usually gives a mixture which has a melting point at a temperature lower than or about the same as the melting point of the component having the lower melting point, while the clear point is between the clear points of the components. Such components having higher melting points and clear points which have been used hitherto are, for example, 4,4"-di-substituted phenyl p-benzoyloxybenzoates of Formula (III) ##STR2## or the biphenyl ester derivatives of Formula IV ##STR3## wherein R and R' are alkyl or alkoxy and X is carbonyloxy. The compounds of Formula (III) have not, however, been employed in a wide field of application because they impart a high viscosity to the liquid crystal dielectrics containing them; as a result, the switching times of the liquid crystal display elements produced with these compounds are prolonged in an undesirable manner. It is true that this effect is not so pronounced when the biphenyl esters of Formula (IV) are used, but these compounds, in particular at low temperatures, are not as readily soluble in the most important liquid crystal base materials as is desirable if the clear point is to be raised.